U.S. patent number 3,611,031 [Application Number 05/045,460] was granted by the patent office on 1971-10-05 for series sequential circuit breaker.
This patent grant is currently assigned to Hughes Aircraft Company. Invention is credited to Michael A. Lutz.
United States Patent |
3,611,031 |
Lutz |
October 5, 1971 |
SERIES SEQUENTIAL CIRCUIT BREAKER
Abstract
The circuit breaker for high-voltage, high-current DC circuits
comprises at least two serially connected transfer switches, an
electronic switch connected in parallel across each of said
transfer switches, an energy absorbing resistor connected in
parallel across each electronic switch except the last electronic
switch so that successive opening of the first transfer switch and
first electronic switch, followed by successive opening of the
remaining transfer switches and electronic switches, causes current
reduction and subsequent interruption.
Inventors: |
Lutz; Michael A. (Los Angeles,
CA) |
Assignee: |
Hughes Aircraft Company (Culver
City, CA)
|
Family
ID: |
21938012 |
Appl.
No.: |
05/045,460 |
Filed: |
June 11, 1970 |
Current U.S.
Class: |
361/9; 361/58;
218/143 |
Current CPC
Class: |
H01H
33/596 (20130101); H02H 3/021 (20130101) |
Current International
Class: |
H01H
33/59 (20060101); H02H 3/02 (20060101); H02h
007/22 () |
Field of
Search: |
;317/11A,11C,11E
;307/133,136 ;200/144AP |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Trammell; James D.
Claims
What is claimed is:
1. A DC circuit breaker comprising:
a DC electric line, first and last transfer switch contacts
serially connected in said DC electric line to open said DC
electric line;
first and last serially connected electronic switches each
connected in parallel to said transfer switch contacts;
a first energy-absorbing resistor connected in parallel around said
first electronic switch;
a current surge absorbing capacitor connected in parallel around
said last electronic switch; and
control means connected to said transfer switch contacts and to
said electronic switches for opening said first transfer switch
contacts while said last transfer switch contacts are closed to
cause current flow through said first electronic switch, and for
subsequently causing said first electronic switches to become
nonconductive to cause current flow through said first
energy-absorbing resistor, and for subsequently opening said last
transfer switch contacts to cause current flow through said last
electronic switch and then causing said last electronic switch to
become nonconductive to cause current flow into said
surge-absorbing capacitor to interrupt current flow in said DC
line.
2. The breaker of claim 1 wherein there are first, second and last
serially connected transfer switch contacts; first, second and last
serially connected electronic switches each connected in parallel
to its respective transfer switch and first and second
energy-absorbing resistors respectively connected in parallel with
said first and second electronic switches.
3. The breaker of claim 2 wherein said first and second
energy-absorbing resistors are nonlinear resistors.
4. The breaker of claim 3 wherein an energy-absorbing resistor is
connected in series with said surge-absorbing capacitor to absorb
system energy upon said third electronic switch becoming
nonconductive.
5. The breaker of claim 4 wherein fault-sensing means is connected
to said DC line to detect a fault therein, said fault-sensing means
being connected to said control means so that upon sensing a fault
in said DC line, said control means causes said interrupter to
interrupt current in said DC line.
6. The breaker of claim 4 wherein said electronic switch is a
crossed field switch.
7. The breaker of claim 4 wherein said electronic switch is a
liquid metal cathode switch tube.
8. The breaker of claim 1 wherein fault-sensing means is connected
to said DC line to detect the fault therein, said fault-sensing
means being connected to said control means so that upon sensing
the fault in said DC line, said control means causes said
interrupter to interrupt current in said DC line.
9. The breaker of claim 8 wherein said electronic switch is a
crossed field switch.
10. The breaker of claim 8 wherein said electronic switch is a
liquid metal cathode switch tube.
Description
BACKGROUND OF THE INVENTION
This invention is directed to a series-sequential circuit breaker
of such nature as to permit the controlled interruption of current
flow in high-current, high-voltage DC power lines.
The multiphase transmission of electric power at a frequency
corresponding to the generating source and the load equipment is
widely used at present. The employment of alternating current is
desirable because it permits the use of transformers to change
voltages from a value suitable for generation, to a value suitable
for transmission, to a value suitable for distribution, and finally
to a value suitable for use.
Increasing power demand by the technologically advancing community
has resulted in transmission at higher voltages and for longer
distances. The transmission line reactance is such that further
increase of transmission line length or voltage becomes uneconomic.
The user must pay for even greater power losses as distances and
voltages are increased.
As a result of this, efforts have been made to transmit electric
power by direct current links. Direct current is much more
satisfactory from a reactance viewpoint for subsea or subterranean
installations. Thus, modern interisland ties have been DC in
nature. The same considerations also apply to long overground
installations. With increasing size of urban centers, and with the
aesthetic demands that lines be placed underground wherever
possible, it is expected that future urban transmission lines will
to a great extent be subterranean. This requirement points to the
need for employing direct current transmission.
Of course, the resistance loss of a DC line is decreased by
increasing the voltage and decreasing the line current. However,
switching and interrupting devices for such higher voltage, and
especially high-current DC transmission lines have previously been
unavailable.
SUMMARY
In order to aid in the understanding of this invention, it can be
stated in essentially summary form that it is directed to a
series-sequential circuit breaker wherein at least two transfer
switches are serially connected in a high-voltage, high-current DC
line, with an electronic switch connected in parallel across each
of said transfer switches and an energy-absorbing resistor
connected in parallel with each of said electronic switches except
the last one. By sequentially opening the first transfer switch and
first electronic switch followed by successive opening of the
remaining transfer switches and electronic switches, DC circuit
current is reduced and interrupted.
Accordingly, it is an object of this invention to provide a
series-sequential circuit breaker which is capable of breaking a
high-voltage, high-current DC circuit. It is a further object to
provide a circuit breaker which will permit the employment of DC
transmission lines with circuit breakers therein to control circuit
faults.
It is another object to provide a circuit breaker having a
plurality of serially connected switches in the DC line with an
electronic switch in parallel to each of them, so that as each
switch in the line is opened, the current is transferred to its
parallel electronic switch, which minimizes arcing of the in-line
transfer switches. It is a further object of this invention to
provide resistances in parallel across all but the last of the
electronic switches so that when the electronic switches are caused
to be nonconductive, the current is forced to flow through the
resistances, which thus reduce the line current. It is another
object to employ nonlinear resistances in parallel to the
consecutively opening electronic switches so that a maximum amount
of circuit energy can be absorbed per electronic switch to limit
the number of in-line transfer switches and limit the number of
electronic switches in parallel thereto. It is a further object to
provide a circuit which employs electronic switches which can be
caused to be nonconductive or caused to interrupt current flowing
therethrough, so that the connection of these electronic switches
in parallel to mechanical transfer contacts causes minimization of
the arcing at the transfer switch contacts.
Other objects and advantages of this invention will become apparent
from a study of the following portion of the specification, the
claims and the attached drawings. Adding a brief description of the
drawings:
FIG. 1 is a schematic drawing of a high-voltage, high-current DC
circuit having the series sequential circuit breaker of this
invention connected therein.
FIG. 2 is a graph of circuit current vs time during the opening
sequence of the circuit breaker of this invention.
FIG. 3 is a graph showing the voltage across the circuit breaker
during the sequence shown in FIG. 2.
FIG. 4 is a diagrammatic showing of the control equipment which
controls the in-line switches and electronic switches of the
circuit breaker of this invention
DESCRIPTION
FIG. 1 illustrates a high-voltage DC circuit with the
series-sequential circuit breaker of this invention incorporated
therein. The circuit is generally indicated at 10 and the circuit
breaker is generally indicated at 12.
The circuit 10 comprises positive bus 14 and return bus 16.
Connected therebetween is a high-voltage, high-current DC power
source 18 which is conveniently illustrated as being a battery.
However, as is well known to those in the art, the power source
usually comprises an engine or turbine driven multiphase AC
generator which supplies power to transformers. The transformers
increase the voltage and supply the rectifiers which are connected
between positive bus 14 and return bus 16. The preferred example
given in this specification is for a 400-megawatt system, because
that power level appears to be appropriate for future use in power
generation adjacent urban environments. Throughout this
specification this example is used to illustrate and describe the
invention. The invention is not limited by the example.
Additionally, the values given as exemplary are applicable only to
the example. When the invention is used in different circuits, the
values will depend on circuit conditions. The example employed as
illustrative of the use of the invention is at a level which might
be used in underground transmission of power from nearby generating
plants to urban areas. In such an example, the normal current is
1,000 amperes, as illustrated by the ordinate in FIG. 2 where each
of the numerals indicates 1,000 a. Furthermore, the normal voltage
level between the positive bus 14 and return bus 16 is 200
kilovolts, as illustrated by the ordinate in FIG. 3 where the
numbers illustrate 1,000 volts. Furthermore, return bus 16 is
preferably at ground potential and a duplicate circuit 10 is
provided with a negative bus at - 200 kv. and a duplicate of the
circuit breaker 12. In other words, FIG. 1 illustrates half of an
exemplary 400-MW system.
Inductance 20 is serially connected with power source 18.
Inductance 20 represents the inductance of the entire circuit. The
circuit inductance limits the change in current with respect to
time, and should the normal circuit inductance be too low, and
additional inductor can be installed for smoothing and for limiting
the rate of current increase in fault conditions. In the specific
example of this specification, the circuit inductance is one-half
henry so that at the 200 kv. power source voltage, the rate of
change of current with respect to time upon occurrence of a fault
is 400 a. per millisecond. Capacitor 19 illustrates system
capacitance.
Serially connected in the line is fault sensor 22, transfer switch
contacts 24, 16 and 28 and load 30. Load 30 can be any conventional
commercial load or any special load which employs the power
produced by the power source. Thus, load 30 can include inverters,
transformers and distribution equipment to the ultimate load. Lines
32 and 34 represent transmission portions of the positive bus 14
and return bus 16, respectively, which transmit the power from the
source to the load. Thus, the circuit breaker 12 is preferably
adjacent the source 18 and transmission over a distance occurs in
lines 32 and 34.
Connection 36, with its switch 38, between lines 32 and 34,
represents a short circuit such as might occur at the input to load
30 or in the lines 32 and 34 leading thereto. Closure of the switch
38 represents an inadvertent short circuit and thus, connection 36
with its switch is schematically illustrative of other types of
highly conductive electrical connections between lines 32 and
34.
Lines 40 and 42 are connected to bus 14 on opposite sides of
transfer switch contacts 24. Electronic switch 44 is connected
therebetween to be in parallel with the contacts 24. Additionally,
energy-absorbing resistor 46 is connected between lines 40 and 42
to be in parallel to the electronic switch 44.
Similarly, lines 42 and 48 are connected to bus 14 on opposite
sides of transfer switch contact 26. Connected between lines 42 and
48, in parallel to contacts 26 are electronic switch 50, surge
capacitor 52 and energy-absorbing resistor 54.
Lines 48 and 54 are connected to bus 14 on opposite sides of
transfer switch contacts 28. Connected between lines 48 and 54 are
electronic switch 56, surge capacitor 58 and the series combination
of surge energy resistor 60 and surge capacitor 62.
Referring to FIG. 4, fault sensor 22 is connected to control unit
64 which contains control circuitry to function as is hereinafter
described. The output of control unit 64 goes to transfer switch
operators 66, 70 and 74 and to electronic switch operators 68, 72
and 76.
Transfer switch operators 66, 70 and 74 are respectively connected
to operate transfer switch contacts 24, 26 and 28. Similarly,
electronic switch operators 68, 72 and 76 are respectively
connected to operate electronic switches 44, 50 and 56.
Fault sensor 22 is any convenient and conventional fault sensor
which is responsive to voltage between buses 14 and 16, is
responsive to current in bus 14 or is responsive to the change in
current with respect to time in bus 14, or a combination of these
signals. Suitable fault sensors are shown in the following U.S.
Pat. Nos. 3,353,171; 3,419,791; 3,463,998; 3,471,784; 3,473,106;
3,475,653; 3,478,352; and 3,489,920. Any one or more of these can
be employed as fault sensor 22. Additionally or alternatively,
manual-actuating means could be employed to actuate the breaker 12
by actuating control unit 64. In some cases it may be desirable to
operate breaker 12 through only a part of the opening sequence. The
control unit 64 can be arranged for such control. The particular
fault sensor is not critical to the invention and any conventional
fault-sensing means can be employed.
Transfer switch contact operator 66 and its contacts 24 are in the
nature of those found in a conventional circuit breaker such as
shown in Waghorne, et al. U.S. Pat. No. 3,268,687. The requirements
are that the transfer switch contacts 24 be able to carry 1,000 a.
when closed (the maximum normal current in the exemplified DC
circuit of this specification), and to withstand the fault current
without damage. Also when open they must withstand without
conduction the surge voltage of the circuit. For the purpose of the
example, the surge voltage is selected to be 1.7 times the normal
circuit voltage. With the normal circuit voltage at 200 kv., the
surge voltage is 340 kv. in accordance with this example. Thus,
when opened and deionized, the transfer switch contacts 24 must be
able to withstand an applied DC voltage of 340 kv. Transfer switch
contact operator 70 and 74 and their respective contacts 26 and 28
are respectively identical to operator 66 with its contacts 24.
The electronic switch 44 can be either a crossed field switching
device, a liquid metal cathode-switching device, both of which are
described in detail in Pat. application Ser. No. 681,632, filed
Nov. 9, 1967, now U.S. Pat. No. 3,534,226 . Other types of
electronic switches which can open with adequate recovery rate and
which can operate under the specified conditions can be employed.
The requirement of the electronic switch 44 is that it be able to
turn on as voltage is applied thereacross. As transfer switch 24
opens, an arc is created thereacross, and as the arc lengthens,
voltage rises across the contacts. When this voltage reaches the
conduction voltage of electronic switch 44, the electronic switch
conducts. As the contacts of transfer switch 24 further open, the
arc extinguishes.
With respect to conduction, it must be able to conduct up to four
times the normal circuit current. In accordance with the example of
the specification, the maximum current has been chosen to be
limited to four times normal current, which is consistent with
surge voltages of 1.7 times normal voltage, and a
one-half-henry-system inductance. Thus, electronic switch 44 must
be capable of conducting up to 4,000 a.
Furthermore, the electronic switch 44 must be capable of
offswitching against this current. In order to be satisfactory for
operation in the circuit of this example, the increase in voltage
withstood by switch 44 with respect to time should be about 1 kv.
per. microsecond. The crossed field switch and the liquid metal
cathode switch of the above-identified patent are satisfactory for
this purpose. Of course, the electronic switch 44 may represent one
or more serially connected electronic switches as described in the
patent, to provide the desired standoff voltage for off-switching
capability should the characteristics of electronic switch devices
of commercial configuration so indicate. The electronic switches 50
and 56 are identical to the electronic switch 44. As is described
in the above identified patent, these electronic-switching devices
can be controlled for on and off switching.
Resistors 46 and 54 are shown as being nonlinear resistors. Such
are preferable, for with nonlinear resistors the circuit breaker 12
of this invention is able to accomplish the circuit-breaking
function of the example with only the two resistors 46 and 54 with
then parallel electronic switches 44 and 50. If linear resistors
were employed instead of a nonlinear resistors 46 and 54, at least
one additional electronic switch would be required. Resistors 46
and 54 are silicon carbide devices. In this example, resistor 46
has such a value that it produces a voltage drop of 340 kv. at
4,000 a. and resistor 54 produces a voltage drop of 130 kv. at
2,500 a.
Surge capacitor 62 is of conventional oil-filled character and has
a value of 2 microfarads in the example of the specification. It is
capable of withstanding the 340 kv. voltage to arrest the final
voltage surge. Its surge suppression resistor 60 has a value of 100
ohms and is capable of carrying 1,000 a. in surge suppression
duty.
In normal operation of circuit 10, power source 18 is supplying
1,000 a. of current through inductance 20, fault sensor 22, closed
contacts 24, 26 and 28 and through load 30. The voltage drop across
the load is the nominal circuit value of 200 kv.
Under these circumstances the electronic switches 44, 50 and 56 are
nonconductive, but are in standby condition so that they will
become conductive when appropriate voltage is applied
thereacross.
At point a in time, a fault appears short-circuiting lines 32 and
34, as represented by the closing of switch 38. This fault causes a
drop in voltage across the load to near zero, and an increase in
current as limited by the value of inductance 20. Current increases
at the rate of 400 a. per msec., as previously described, sensor 22
senses the increase in current, the rate of increase of current, or
the decrease in voltage between buses, or the combination of these
signals to determine that a fault has occurred. Such determination
occurs at point b along the abscissa of the graphs of FIGS. 2 and
3. As indicated, FIG. 2 represents the current through sensor 22,
which is equal to the current through power source 18. FIG. 3
represents the voltage across breaker 12. It represents the voltage
drop which is produced by the opening of the breaker. In the
graphs, from the intersection to point a, current is normal and
voltage drop across the breaker is zero. At point a the short
circuit occurs. In the time interval from a to b, the sensor makes
the decision as to whether or not it should actuate, and operator
66 opens contacts 24 at point b. The voltage drop produced by the
initial arcing causes conduction of electronic switch 44 which had
been on standby condition awaiting such a voltage applied
thereacross, so that through the interval from b to c, the voltage
drop across the breaker comprises the voltage drop in electronic
switch 44 when it is conducting (slightly reduced by the parallel
circuit through resistor 46). From the time interval from b to c,
contacts 24 are opened and deionized.
When deionization is complete, at point c operator 68 of electronic
switch 44 causes off-switching of the electronic switch. This
places the resistance 46 in the line. The value of the resistance
is chosen so that at this peak current the placement of that
resistance in the line does not cause a surge greater than
tolerable, in this case to 340 kv. Should the system capacitance 19
be inadequate to aid in limiting this peak voltage, an additional
capacitance can be provided in parallel with resistor 46. This
resistance causes reduction in current, with consequent reduction
in voltage drop across the breaker.
When this voltage drop across the breaker reduces to near 200 kv.,
the normal line voltage, operator 70 opens transfer switch contacts
26. This opening causes a voltage drop across electronic switch 50
so that it becomes conductive. When contacts 26 are fully opened
and deionized, operator 72 turns off electronic switch 50 at point
d. This again causes an increase in voltage drop across the breaker
by inserting energy-absorbing resistor 54 in series with resistor
46. The rate of voltage rise is limited by capacitor 52 in addition
to the already inserted system capacity. Values are chosen so that
the voltage increase by off-switching of electronic switch 50 does
not exceed the allowable 340 kv. chosen for this example.
This series combination of resistors causes a further reduction in
current through the time period from d to e, together with a
reduction in voltage drop across the breaker from near the maximum
allowable value toward the nominal line voltage, as indicated in
FIG. 3.
In the final step, controller 64 causes the opening of transfer
switch contacts 28 which causes voltage drop across the electronic
switch 56 which was in standby condition. With this voltage drop,
the electronic switch becomes conductive. Transfer switch contacts
28 are fully opened and deionized. At this point, controller 76
causes off switching of electronic switch 56 at point e. The final
surge current is absorbed in capacitor 62 with its energy-absorbing
resistor 60. When the surge has subsided in the capacitor, the
current through the sensor is reduced to zero and the voltage drop
across the breaker 12 is at line voltage. The circuit breaker 12
can be employed as a main switch for opening the bus, either at the
source and/or load end thereof. Furthermore, it can be employed as
a switch for a branch line on a transmission line. Thus, the
circuit breaker 12 is a special purpose application of a generic
switch.
This invention having been described in its preferred embodiment,
it is clear that it is susceptible to numerous modifications and
embodiments within the ability of those skilled in the art and
without the exercise of the inventive faculty.
* * * * *